The present invention relates to rotary drilling tools and, more particularly, relates to roof drill bits for use in coal mines.
Most particularly, the present invention relates to two-prong, center vacuum roof drill bits.
Roof drills are used for drilling holes in the roofs of underground coal mines. After the hole is drilled, the drill is removed and an anchor is inserted in the hole as an aid in supporting the mine roof. Under certain conditions, a roof drill bit is selected which can be used in conjunction with a vacuum system. When the vacuum system operates through the center of the bit, it is called a center vacuum bit. The purpose of the vacuum system is to collect the fine particles generated during the drilling of the hole. When this center vacuum roof drill bit becomes worn, it must be replaced.
The center vacuum roof drill bit which has been the standard in the coal mining industry for a number of years is illustrated in FIG. 1. This drill bit 2 can be generally described as an insert 4 brazed 6 to a supporting body 8 wherein the insert comprises a flat piece of hard, wear resistant material (usually cemented carbide). The hard insert has a zero degrees rake angle and a continuous cutting edge profile 10 extending diametrically across the roof drill bit.
There are a number of long-standing problems with this prior art roof drill bit. A distinct disadvantage of a continuous cutting edge profile is the lack of cutting speed in the center of rotation, thereby requiring higher thrust at this point to penetrate the rock. The higher thrust required may cause the bit to fail, especially when drilling in hard rock formations.
Another problem with the prior art roof drill bit is the cutting action of the bit. The prior art roof drill bit proceeds through the rock by first primarily crushing the rock, then penetrating into the crushed rock and finally shearing the rock. This type of action requires a high thrust with a concomitant increase in wear of the hard, wear resistant material. Typically, the prior art roof drill bit is replaced after only one pass.
A further problem with the prior art roof drill bit is the generation of very fine dust particles, which causes the vacuum system to work harder than is necessary. Those particles which escape the vacuum system become a health hazard to those in the mine.
The applicant of the present invention is familiar with another type of rotary drill bit which is illustrated in U.S.S.R. Inventor's Certificate No. 395271. The present applicant is a coinventor of the drill bit disclosed in this document. The rotary drill bit illustrated in this Inventor's Certificate was also the subject of a paper authored by the applicant. This paper was entitled "Hand-Held Power Tools for Drilling Small Diameter Holes in Construction Materials," Industrial Construction and Engineering Structures Magazine, Kiev (USSR), Volume 1 (1975).
The rotary drill bits illustrated in the Inventor's Certificate and in the paper are masonry drill bits for drilling in concrete, brick or other similar construction materials with a hand-held electric drill. These masonry drill bits are tipped with wedge-shaped inserts. A comparison of the operating conditions of masonry drill bits and roof drill bits illustrates the fact that the suitability of a drill bit for use as a masonry drill bit will not necessarily indicate that this same drill bit is suitable for use as a roof drill bit. The comparison of masonry and roof drill bit operating conditions is as follows:
______________________________________ Masonry Bit Roof Bit ______________________________________ Cutting Diameter .5 or less 1 or more Inches Thrust, Pounds 40-50 1500-7500 Speed, rpm 700-1000 250 Power at Motor, HP .25-.35 3-5 Torque, Foot-Pounds 1.3-2.5 60-80 ______________________________________
It is immediately obvious from this comparison that the thrust, power and torque for the respective masonry and roof drill bits differ by orders of magnitude.
Accordingly, it is an object of the invention to have a roof drill bit, and insert therefor, without any of the disadvantages of the prior art roof drill bits.
It is another object of the invention to have a roof drill bit and insert therefor, which achieves the optimum in strength and resistance to chipping and fracturing.
It is another object of the invention to have a roof drill bit, and insert therefor, that can drill faster at lower thrust levels and suffer less wear than the prior art use of drill bits.
It is still another object of the invention to have a roof drill bit, and insert therefor, that is economical to manufacture and cost effective in use.